When uploading a file to a remote site, filesize should be used to tell
libcurl what the expected size of the input file is. This value must be passed
as a long. See also CURLOPT_INFILESIZE_LARGE(3) for sending files
larger than 2GB.

When sending emails using SMTP, this command can be used to
specify the optional SIZE parameter for the MAIL FROM command.

This option does not limit how much data libcurl will actually
send, as that is controlled entirely by what the read callback returns, but
telling one value and sending a different amount may lead to errors.